The present invention relates to a method applicable to an electrophotographic copier or similar image recorder for controlling the density of an image in such a manner as to maintain it constant at all times.
In the above-described type of image recorder, a latent image electrostatically formed on an image carrier by a predetermined procedure is developed by a toner, i.e., colored fine particles fed from a developing device. The toner is usually charged to the opposite polarity of the latent image and electrostatically deposited on the latent image. To charge the toner to such a polarity, it may be combined with a carrier to constitute a two-component developer and agitated together with the carrier for frictional charging. While this kind of development using a two-component developer is capable of charging the toner sufficiently, the toner concentration sequentially decreases since only the toner is consumed during development. Therefore, the toner concentration of the developer, i.e., the density of an image to be developed by the toner has to be controlled to a predetermined value. This may be done by measuring the current toner concentration of the developer and, based on the measured toner concentration, controlling the amount of toner supply, i.e., the amount of toner to be fed to the carrier.
The toner concentration of the developer may be directly determined in terms of the weight or the permeability of the developer. Such direct measurement may be replaced with indirect measurement which uses a white reference pattern and a black reference pattern. Specifically, for the indirect measurement, latent images representative of a white and a black reference pattern are electrostatically formed on a photoconductive element and developed by a developer. The densities of the resulting toner images are measured by a photoelectric arrangement. More specifically, a photosensor or so-called P sensor is located in close proximity to the surface of the photoconductive element to sense the densities of the toner images of the reference patterns, so that a particular amount of toner supply is selected on the basis of the ratio of the sensed densities. This kind of scheme, therefore, determines a change in the density of each toner image of interest in terms of a change in the toner concentration of the developer, i. e., the mixture ratio of toner and carrier. An electrophotographic copier, for example, using such a method effects the measurement once every time ten copies are produced.
The conventional control method using a P sensor as stated above has the following problems left unsolved.
(1) Since the toner supply begins only after the toner concentration has lowered, the toner concentration sharply changes when documents of the kind consuming much toner are continuously copied, preventing the toner concentration from remaining constant.
(2) Since no consideration is given to the interval between the supply of toner and the resulting increase in toner concentration, the toner concentration is scattered over a broad range, i.e., the control accuracy is not satisfactory.
(3) Toner images representative of the reference patterns are formed once every ten copies without exception, as stated earlier. Hence, when a document of the kind consuming a relatively small amount of toner is copied a plurality of times, it is likely that a greater amount of toner is consumed by the toner images of the reference patterns than by the images of the document. On the other hand, when documents to be sequentially copied are of the kind consuming a great amount of toner, the conventional control method cannot accurately follow the change in the amount of toner.
Moreover, with the conventional image density control method, it is impossible to supply a toner in an amount accurately matching the toner consumption at all times since the amount of toner consumed during the intervals wherein the toner images of the reference patterns are not formed noticeably changes depending on the pixel density of a document and varying ambient conditions. Specifically, changes in the pixel density occurring during such intervals (stated another way, changes in the amount of toner consumption) disturb the photosensor output feedback line. This might be compensated for if the toner images of the reference patterns were formed more frequently to increase the amount of feedback. However, such an approach would aggravate the wasteful toner consumption and increase the load to act on a cleaning unit.
Japanese Patent Laid-Open Publication No. 33704/1988 teaches a scheme using first detecting means for detecting the amount of toner consumed by counting image forming signals, and second detecting means for detecting the amount of toner scattered around by determining the operation time of a developing roller. This scheme maintains the toner concentration constant by supplying a toner in response to the outputs of the two detecting means. However, the problem is that the ability of a developing unit changes since the relation between the image forming signal and the amount of toner consumption changes with a change in the charging ability of the carrier which is ascribable to the deterioration of the developer due to the varying environment. As a result, it is difficult to maintain the ideal image quality (toner concentration) overcoming the varying ambient conditions.
Generally, a two-component developer applicable to, e.g., an electrophotographic copier sequentially reduces the above-mentioned charging ability due to deterioration ascribable to aging. In addition, in a low temperature and low humidity environment, the charge accumulation degree and, therefore, Q/M increases; in a high temperature and high humidity environment, the charge leak degree and, therefore, Q/M decreases. It has been customary to determine a control value by considering only the influence of one or two factors individually. This is contradictory to the fact that many correlated factors effect Q/M, i.e., an optimal control value has to be determined in consideration of many pluralistic information, as mentioned above. Consequently, changes in environment cannot be coped with, preventing high image quality from being maintained.
Regarding image density, it is a common practice for the operator to manually select a desired image density by entering ambiguous "light" or "dark" information in terms of a quantized stepwise numerical value. As a result, the actual image density is not always identical with the desired one.
As for a toner supply member, when the amount of toner remaining in a toner hopper changes, the transport efficiency of the supply member changes. Hence, the toner cannot be supplied in a constant amount when the toner supply condition is maintained the same.
Assume that a toner is consumed continuously in a great amount and the supply of a great amount of toner is needed, as when a black solid image is formed. Then, the target image density cannot be easily maintained since the amount of toner sharply changes.
When the deterioration of a toner is detected, it has been customary to simply control the image density without giving consideration to the cause of the deterioration. This is not always satisfactory in effecting optimal control.
In addition, a conventional image recorder has a temperature sensor and a humidity sensor responsive to the environment and disposed within a developing unit thereof. In this condition, the toner is apt to smear the sensors to lower their sensing ability.